Generally, in a radio technology such as a technology underlying digital communications or a technology underlying digital broadcastings, a signal transmitted by a signal transmitting apparatus through a variety of transmission lines is received by a signal receiving apparatus. Typical examples of the signal transmitting apparatus are a broadcasting apparatus of a broadcasting station, a portable terminal and a relay station whereas typical examples of the signal receiving apparatus are a TV receiver, a portable terminal and a relay station. In a multipath environment in which a signal is transmitted by a signal transmitting apparatus through a variety of transmission lines to a signal receiving apparatus, the signal to be received by the signal receiving apparatus includes reflected waves and echos as described below.
For example, when a signal transmitted from a signal transmitting antenna connected to a signal transmitting apparatus 11 is received by a signal receiving antenna connected to a signal receiving apparatus 12, the signal receiving apparatus 12 actually receives a principal wave directly transmitted by the signal transmitting apparatus 11 and a wave reflected by the building 13 or the like to appear as a delayed wave delayed by a building 13.
Thus, a signal actually received by the signal receiving apparatus 12 is a compound wave including the principal wave and the delayed wave as described above. That is to say, a signal actually received by the signal receiving apparatus 12 is a distorted wave. Thus, the quality of a communication carried out in a multipath environment deteriorates. Therefore, in order to improve the quality of a communication carried out in a multipath environment, it is necessary to grasp characteristics of the multipath environment. Generally determined by the communication environment, the characteristics of the multipath environment include the transmission distance of each of a plurality of transmission lines composing the communication environment and the reflection characteristic of each of the transmission lines. In the following description, the characteristics of a multipath environment are properly referred to as a channel profile.
In a process of estimating a channel profile, a synchronization signal inserted into a data frame of a transmitted/received signal is used. The synchronization signal is a series of known symbols. The signal receiving apparatus 12 has also registered the series of known symbols in advance. In the following description, the symbol series inserted into a received signal is also referred to as the symbol series of the received signal whereas the symbol series registered in the signal receiving apparatus 12 in advance is also referred to as the symbol series of the synchronization signal. The signal receiving apparatus 12 computes the correlation value between the synchronization signal inserted into the principal wave of the received signal and the registered series of known symbols, estimating a channel profile by making use of the computation result which is referred to hereafter as correlation data.
As an example, FIG. 2 is given as a plurality of diagrams referred to in description of a typical channel profile estimated from a signal received by the signal receiving apparatus 12 in a multipath environment. The horizontal axis of the diagram of FIG. 2 represents the lapse of time t. In this case, the time t lapses in the left-to-right direction. Each of arrows erected in the upward direction represents the magnitude (or the level) of a correlation value.
As shown in the diagram of FIG. 2A, the signal received by the signal receiving apparatus 12 includes a principal wave and a post-echo. As shown in the diagram of FIG. 2B, on the other hand, the signal received by the signal receiving apparatus 12 includes a principal wave and a pre-echo.
By acquiring such a channel profile, the signal receiving apparatus 12 is capable of grasping the characteristics of a transmission line through which the signal received by the signal receiving apparatus 12 has been transmitted by the signal transmitting apparatus 11. For example, the signal receiving apparatus 12 is capable of grasping information used for determining whether or not the signal received by the signal receiving apparatus 12 has been transmitted by the signal transmitting apparatus 11 through a transmission line which generates a post-echo or a pre-echo. Thus, when carrying out a communication in a multipath environment, by performing equalization processing based on the channel profile of the multipath environment, it is possible to acquire the basic waveform from a distorted waveform. That is to say, the waveform of only the principal wave can be obtained. Thus, the quality of the communication can be improved.
Next, processing to estimate a channel profile is explained by referring to a diagram of FIG. 3.
For example, for every data frame having a frame length determined in advance, the signal transmitting apparatus 11 transmits a signal including a synchronization signal inserted into the head of the data frame to the signal receiving apparatus 12 in a multipath environment. In this case, the signal receiving apparatus 12 actually receives a compound signal including the principal wave of the original signal and a wave delayed due to effects of the multipath environment. The signal receiving apparatus 12 has registered the synchronization signal inserted into the signal received from the signal transmitting apparatus 11 as a series of known symbols in advance. The signal receiving apparatus 12 computes the correlation value representing a correlation between the synchronization signal inserted into a received signal and the registered series of known symbols from time to time while sliding the symbol series of the synchronization signal over the symbol series of the received signal.
In the case of a received signal including a principal wave and a delayed wave as described above, in a channel profile found on the basis of correlation data representing computed correlation values, a correlation value computed with a timing coincident with the synchronization-signal of the principal wave is the largest value and a correlation value computed with a timing coincident with the synchronization signal of the delayed wave is the second largest value. A correlation value computed with a timing other than the timing coincident with the synchronization signal of the principal wave and the timing coincident with the synchronization signal of the delayed wave is relatively small.
As shown in a diagram of FIG. 4, the signal receiving apparatus 12 compares the correlation value computed from time to time with a correlation threshold value th determined in advance and stores the correlation value in a memory. Strictly speaking, if the correlation value is found equal to or greater than the correlation threshold value th for the first time, the signal receiving apparatus 12 starts an operation to store correlation values in a memory. The memory has N storage locations which are given addresses 0 to (N−1) respectively. The first correlation value found equal to or greater than the correlation threshold value th for the first time is stored at a storage location with an address of 0. In this case, the subsequent correlation values following the first correlation value are stored at the storage locations with addresses of 1 to (N−1).
If the received signal includes a post-echo for example, when the principal wave of the received signal is detected, the signal receiving apparatus 12 starts an operation to store correlation values in the memory shown at the bottom of the diagram of FIG. 4. The correlation value computed at the detection time of the principal wave is stored at an address of 0 in the memory as shown by a hatched block on the left end of the memory. Detection of the post-echo is delayed from the detection of the principal wave by a delay time. If the delay time is shorter than a period having a length corresponding to the N storage locations of the memory, the correlation value computed at the detection time of the post-echo is stored in the memory as shown by a hatched block in the middle of the memory. The computed correlation values stored in the memory having a size N storage locations as correlation values including the correlation value of the principal wave and the correlation value of the post-echo are referred to as correlation data from which a channel profile is estimated.
If the process of setting the correlation threshold value th to be compared with the computed correlation values is not carried out properly, however, a proper channel profile cannot be acquired, for example, from the correlation data including the correlation value of the principal wave and the correlation value of the post-echo as shown in the diagram of FIG. 4.
FIG. 5 is a diagram showing a typical case in which the correlation threshold value th is set at a level higher than a proper one. In this case, the correlation value of the principal wave is computed, being compared with the correlation threshold value th and an operation to store computed correlation values in the memory is started. Even if the received signal includes a pre-echo leading ahead of the principal wave, however, the computed correlation value of the pre-echo is not stored in the memory.
In order to allow the computed correlation value of the pre-echo to be detected and stored in the memory, on the other hand, the correlation threshold value th is set at a low level. FIG. 6 is a diagram showing a typical case in which the correlation threshold value th is set at a level lower than a proper one. In this case, the correlation value computed for a noise to appear as a correlation value higher than the correlation threshold value th is detected and an operation to store computed correlation values in the memory is started with an operation to store the correlation value computed for the noise. As shown in the diagram of FIG. 6, even though the computed correlation values of pre-echos and the principal wave can be stored in the memory, the computed correlation value of a post-echo cannot because the pre-echo is detected at a time beyond a period corresponding to the N storage locations which form the size of the memory.
In the typical cases described above by referring to the diagrams of FIGS. 5 and 6, it is impossible to obtain correlation data including the required correlation value of a post-echo or a pre-echo albeit including the correlation value of the principal wave. Thus, a proper channel profile cannot be estimated.
In addition, Japanese Patent Laid-open No. 2004-343542 describes a communication terminal in which an averaged-correlation-value profile is found by averaging the values of correlations between a received signal and a reference signal in order to detect a multipath in radio transmission lines.
As described above, a proper channel profile cannot be estimated from acquired correlation data in some cases.